Abstract

Purpose:

Identifying predictors of translational and rotational offsets for liver SBRT treatment may be of assistance in determining if the use of a 6D couch is more advantageous for particular patients.

Methods:

From February 2013 to February 2015, 85 patients received liver SBRT for HCC or metastases. CBCT imaging was used for each patient and offsets recorded. There were 399 initial CBCT registrations performed. 184 registrations contained angular offsets. The mean offsets for patients in the superior/inferior, anterior/posterior, and lateral dimension were 4mm, 4mm, and 3mm, respectively. The mean absolute angular yaw, pitch, and roll were 0.2°, 0.3°, and 0.3°, respectively. Mean GTV, PTV, and Liver volume was 46.6, 153.3, and 1435.2cc's. 59 patients were male and the mean body mass index (BMI) was 27.2kg/m2. Hepatic segments were characterized as anterior/posterior left/right, superior/inferior, or lateral/medial by established anatomic classification. Chi-squared test and Multinomial Logistic Regression were used for univariate (UVA) and multivariate (MVA) analyses.

Results:

Offsets ≥ 2mm, 5mm, and 10mm occurred 82.5%, 49.4%, and 9.5% of the time, respectively. While kV imaging prior to CBCT decreased the frequency of 2 and 5mm offsets on UVA, on MVA BMI's >20 predicted for offsets. On UVA and MVA, lateral lesions were more likely to have ≥ 10mm shift. Offsets ≥ 0.2°, 0.5°, and 1° occurred 76.1%, 68.5%, and 42.9% of the time, respectively. Lateral lesions were more likely on MVA to experience ≥ 0.2° and 0.5° shifts, while previous kV images were the only variable that predicted for shift < 1°. No dosimetric parameter predicted for translation or rotation offsets on MVA.

Conclusion:

BMIs <20 predict for smaller translations during liver SBRT, while lateral lesions predict for greater translation and rotation offsets. Patients treated with lateral lesions might benefit more from the angular capability of a 6D couch.